When a skeletal joint is compromised, whether through injury, pathology, misalignment, overuse, or surgery, bracing is often necessary to provide support and possibly facilitate healing. Conventional external mechanical braces, however, have been shown repeatedly to cause muscle atrophy by supplanting normal muscle activity. In that this effect is antipathetic to the basic principles of rehabilitation, devices which actively stimulate the body's own musculature surrounding the compromised joint are increasingly being used for protection and/or rehabilitation. One of these devices may be worn throughout the day, constantly monitors the patient's movements, and responds to problematic joint circumstances by stimulating muscles in a manner which opposes the incident force causing the problem. A form of muscle stimulation used in these devices is electrical current.
These devices, however, rely upon pre-programmed templates for operational parameters, or learn physical conditions deemed problematic for the joint by either the patient or an attending medical practitioner. They are therefore inherently open-loop systems, responsive solely to physical conditions of the joint, without regard for direct or indirect effects of the dynamic muscle stimulation they provide. Not only does this open-loop nature necessitate programming applicable to a broad range of patients (as opposed to individualized therapy), it as well precludes adaptation by the device to incremental improvements made by the patient through use of the device. No attempt may therefore be made by such as system to regulate therapy toward a nominal state for that particular patient. Application of closed-loop techniques would allow these devices to continuously adapt to the individual patient on an ongoing basis.
The term ‘closed-loop’ is used herein to denote proportional control of a control system output (such as electrical muscle stimulation current), as a linear or non-linear function of one or more error terms. These error terms, as commonly practiced in the art, consist of deviations between a desired value of a measured input (command term) and the actual measured input.
Bones and joints both have been known for some time to exhibit piezoelectric properties. Following the discoveries that bones become stronger in adaptation to stress, and that physical stress induces localized currents in the bone, bone growth stimulators have been developed which apply controlled mechanical stress and/or electrical current to a damaged tissue area. Consistent with observed piezoelectric activity and stress-induced growth of bone and joints, electrical stimulation which imposes a DC bias has shown to accelerate tissue regeneration.
Piezoelectric activity is considered to be a minor contributor to natural electrical currents in and near joints. Each change in skeletal loading causes fluid flow through bone and particularly cartilage. Due to constituent charged particles, this fluid flow creates dynamic electrical currents which impose what are referred to as streaming potentials across the surrounding tissue. Streaming potentials have significance both from a diagnostic perspective, and in their capacity for fluid flow modulation. In addition to possible impact on cartilage hydration in eroded joints through imposed steaming potentials, control of chondrocyte migration has been shown to occur from imposed electrical potentials.
Diagnostic measurement of joint potentials under dynamic loading is taught in U.S. Patent Application Publication No. 20110034797, ‘Non-invasive measuring of load-induced electric potentials in diarthroidal joints’. Neither use of the subject matter of the application outside a diagnostic setting, nor therapeutic modulation of potentials discovered is addressed. Furthermore, the subject matter of the application does not address the relationships between the myriad force vectors possible during normal activity and the resultant streaming potentials. Vectors of incident forces upon a joint become much more significant when applied to joints comprised of multiple load-bearing surfaces.
To date, devices that stimulate bone and cartilage growth through electrical stimulation have relied either upon constant excitation or pre-programmed stimulation sequences. In contrast, piezoelectric activity and streaming potentials during normal patient activities are dynamic—polarities and magnitudes of the currents generated are resultant of incident forces, so constantly follow physical activity. Stimulation devices which are non-responsive to physical activity therefore are incapable of either mimicking or bolstering natural biological piezoelectric or streaming potential activity. In that it has been found that synchronizing muscle stimulation with volitional exertion, it is improved tissue regeneration may result from synchrony between physical stress and stimulation. To compound difficulty in bolstering or supplanting this electrical activity of a specific patient, huge subject response variances have been reported. This strongly implies that broad success of generalized stimulation will be less probable without adaptation to each specific case.
Synchronization of stimulation to the gait cycle, for the purpose of impacting cartilage health, is explored in U.S. Pat. No. 8,060,210, ‘Methods for improving mobility and controlling cartilage matrix degradation of weight-bearing articular joints’. The subject matter of this patent addresses motor-level stimulation of antagonistic muscles in a timed fashion, so as to minimize pressure or moving friction, but makes no distinction between reduced joint forces through muscle contraction and charged particle migration through the joint tissue. In that timing, physical location, and stimulation waveforms required for joint force reduction may or may not differ substantially from those required for fluid flow modulation, the arbitrary application of waveforms before and/or after application of unspecified multiphasic stimulation, as taught therein, does not show independent fluid flow control.
U.S. Pat. No. 7,822,481 addresses adjustment of a therapy program in response to one or more sensed patient parameters, but does not describe stimulation intensity to be any direct function of patient activity or circumstance. Adaptation by stimulators to dynamic physical conditions can be found both in cardiac stimulators and neural stimulators used for pain masking, such as is disclosed in U.S. Pat. No. 7,822,481, ‘Therapy adjustment’. These devices alter one or more parameters of pre-programmed stimulation patterns in response to body position or inclination, activity level, etc. None of these devices, however, stimulate tissue as a direct function of dynamic physical conditions imposed on the stimulated area.